The invention relates to mold blocks, and more particularly to a mold block assembly and methods for forming plastic pipe having transverse profile features.
Manufacturers of corrugated plastic pipe have successfully displaced concrete pipe and corrugated metal pipe in drainage applications. This success initially occurred with corrugated plastic pipe having diameters of less than 30 inches. Supplying a uniform vacuum along the interior lateral surface of the mold and efficiently cooling the molded pipe have been problematic with corrugated pipe having diameters of 30 through 72 inches. The pipe is typically fabricated by co-extruding an inner and outer tube of melted plastic. The inner tube is kept smooth by passing it over a cooling and sizing mandrel. Transverse corrugations are fabricated by thermoforming the continuously extruded outer tube of melted plastic. This thermoforming operation is accomplished by closing mating pairs of mold blocks around the tube of molten plastic, drawing the molten plastic tube into the desired corrugated shape by creating a negative pressure at the interior lateral surface of the mold block, and solidifying the formed molten plastic tube by cooling the plastic to a temperature below its softening point. During this process the mold blocks travel with the molten plastic tube.
Because the mating pairs of mold blocks must constantly repeat a cycle of closing around the extrusion die, traveling with the extruded molten plastic tube, opening and returning to the extrusion die head location, it is not practical to connect a series of hoses and/or tubing for vacuum porting or cooling fluids. As a result, the mold block typically is supplied negative pressure via a single sliding connection. The uniformity of negative pressure at the mold surface therefore depends on the length and size of the vacuum paths contained in the mold block itself. U.S. Pat. No. 5,059,109 to Heindrich Dickhut and John S. Berns and U.S. Pat. No. 5,002,478 to Manfred A. A. Lupke recognized this problem. Both attempted solutions whereby a single sliding vacuum connection per mold block is utilized. The single vacuum connection necessitates variations in the path length from the negative pressure source to locations on the interior lateral surface of the mold blocks. Stated in another way, there will be areas on the mold surface further from the vacuum source than others. As the size of the mold block approaches 60-72 inches in diameter, the distance between the mold surface and the vacuum source can vary as much as 90-115 inches. Newton""s law of viscosity at a constant flow rate predicts a linear dependence of the pressure on the distance the air travels from the mold surface to the vacuum source. This dependence results in a reduction in negative pressure available at locations located far from the vacuum source. Lupke requires that the mold be constructed in segments having a width of one pitch of the corrugation. Such construction requires a great deal of costly precision machining and still does not allow sufficient vacuum porting.
The present invention contrasts with the prior art, which embodies the misconception that in order to minimize the change in pressure and resistance to flow through a pipe or closed channel, the area normal to the flow must be enlarged. The problem with this approach is that the resistance to flow along a path is not simply dependent on the area. Thus, in order to insure uniform vacuum pressure at the internal lateral surface of the mold, it is insufficient to merely determine the dimension of area normal to the flow.
The present invention offers a means of accurately providing a large and uniform vacuum at the mold surface by defining the shape and size of the channel. Dickhut et al. teach a channel with a depth less than its width. Although an intuitive approach assumes the pressure drop along a channel with a larger area is always smaller than that along a channel with a smaller area, this assumption is false. It is much more efficient for channel depth to be larger than channel width because the change in pressure varies with the 3rd power of the channel depth and only linearly with the channel width. A narrower and deeper channel with the same area will be superior to a wider and more shallow channel for purposes of optimizing the force and uniformity of negative vacuum pressure at the mold surface. Similarly, a wider channel with smaller depth might have a larger area than one which presents much less resistance to flow. The present invention quantifies this phenomenon so as to eliminate design ambiguities and provides a criterion for optimizing the performance of the mold blocks.
A second problem that occurs with increased size of mold blocks is the reduction in cooling efficiency caused by the decrease in the ratio of the lateral surface area to the volume of the mold blocks. The exterior lateral area as compared to the volume of the mold block decreases rapidly as pipe diameter increases, as can be demonstrated by the parametric representation of a cylindrical tube:
Volume of a cylinder V=xcfx80(Do2xe2x88x92Di2)L/4
Area of the exterior lateral surface of a cylinder Ao=xcfx80DoL
Ratio of the exterior lateral surface to the volume A/V=4Do/(Do2xe2x88x92Di2)
Where:
Di=inside diameter of a cylinder
Do=outside diameter of a cylinder
L=length of the cylinder
Thus, the heat transfer area available to be cooled compared to the volume conducting the heat away from the internal lateral mold surfaces decreases significantly as the diameter of the mold block increases. Lower cooling rates associated with molds for large diameter corrugated plastic pipe cause quality problems, which are compensated for by increasing the number of traveling mold blocks in contact with the forming pipe or by slowing the extrusion rate of molten plastic pipe. Both options have a significant cost impact on the manufacturer.
It is an object of this invention to provide uniform negative pressure at the interior lateral surface of mold blocks utilized to manufacture large diameter corrugated plastic pipe. This is accomplished by providing mating pairs of mold blocks with open transverse channels fabricated on an exterior lateral surface. The open transverse channels connect a vacuum source to a series of radial slots in the valley of the channel. The series of radial slots in turn intersects a series of thin slits located at the interior lateral surface of the mold block. Each intersection of a radial slot and thin slit forms an orifice having a length that is independent of the depth of the slit. Each slot is of approximately the same width in the preferred embodiment, which ensures that slit length is independent of slit depth.
Each open transverse channel is fitted with an inner air-tight cover forming an exterior manifold that provides a low-resistance path between the vacuum source and the series of radial slots connected to slits located at the interior mold surface. The external manifold must have a cross-sectional area sufficiently large to insure that the change in negative pressure between the vacuum source and the series of slots connected to the slits at the mold surface is very small compared to the change in negative pressure between the intersections of radial slots and thin slits and the interior lateral surface of the mold blocks.
It is a further object of the present invention to provide mold blocks capable of efficiently removing heat to solidify the molten plastic into a pipe with transverse corrugations in a timely fashion. This invention provides enhanced cooling by forced convection so that an outer cover forms a duct with the inner cover and the walls of the open transverse channel. This duct provides a means of directing a stream of turbulent cooling air along the interior walls of the external channel before it exits to the atmosphere. The combination of the radial surface area of the transverse channel and the flow rate of cooling air sufficiently remove in a timely fashion the heat conducted to the mold surface during the process of cooling the molten plastic pipe to its melting point and then crystallizing the plastic and finally cooling the crystallized plastic pipe to increase its stiffness.
This invention relates to the design and assembly of mold blocks and methods for forming large diameter plastic pipe with transverse corrugations and has the following benefits: (1) uniform vacuum porting at the internal lateral mold surface; (2) enhanced heat transfer characteristics; (3) removable covers for inspecting, maintaining and modifying the mold blocks; and (4) elimination of the high fabrication costs associated with cross drilling and with segmented mold blocks having internal channels.